If you were asked to classify every species of vertebrate living in a given environmental niche, how would you go about doing it? For years, ecologists have risen to this task by resorting to brute force — tracking, trapping, and tagging individual animals to extrapolate species population data.

But scientists have now shown that there may be an easier way: just ask the earth. Researchers have recently revealed how easy it is to gain accurate insight into an area's biodiversity by simply searching the soil for traces of animal DNA.

To assess how much information about species population could be gained from so-called "dirt" DNA, researcher Eske Willerslev and his colleagues examined soil samples taken from game parks, zoological gardens, and farms with known numbers of vertebrate species.

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When Willerslev's team extracted the DNA fragments contained in these soil samples and cross referenced them with ones housed in the DNA database GenBank, it found DNA from all but one of the species represented in the various parks, gardens, and farms that they acquired samples from. (The animal whose DNA was not detected was a giraffe that had only recently been introduced to one of the reserves.)

What's more, the researchers found that the relative quantities of a species' DNA in the soil samples gave them a pretty good idea of that species' relative population size.

"This is the first time anyone has shown that 'dirt' DNA not only reflects what species live in an area, but how many [individuals] there are," said Willerslev.

The researchers also found DNA fragments from animals that no longer lived in the test areas, suggesting that the dirt DNA method could even be used to analyze ecosystems past. Willersley, for example, envisions applying the dirt DNA method to deeper soils to shed light on animal populations from thousands of years ago.